At the presidential level, this question won’t be answered until the 2024 Republican primary begins in earnest. But key trends in how younger Republican Senators scored on the Bipartisan Index (BPI) in comparison to their older Senate colleagues point to a shift toward greater partisanship and, perhaps, fealty to forces aligned with Trump. 

During Trump’s presidency, the Lugar Center observed what we called a “Trump effect” in the Bipartisan Index (BPI). This was the phenomenon of rising GOP Senate scores despite broad perceptions that the Trump Presidency had led to unprecedented partisanship on high profile national issues. The theory behind the Trump effect is that GOP Senators perceived dual threats to their electoral survival. If they crossed Trump on any issue that he cared about they risked a backlash from the President’s followers or even a primary. But Trump’s volatile personality and low presidential approval ratings simultaneously threatened to undermine GOP Senators in their general elections. Therefore, they sought to construct cooperative legislative resumes, joining with Democrats on many under-the-radar initiatives.

Irrespective of the Trump effect, many younger GOP senators, who are often looked to as the future of the party, had low BPI scores. For example, despite strongly disagreeing on Trump’s impeachment, Sen. Ben Sasse of Nebraska and Sen. Ted Cruz of Texas ranked #94 and #86 respectively on the latest BPI. Sasse earned a -0.929, while Cruz earned a -0.605, with both senators clocking in near the bottom of the GOP caucus. And yet, it doesn’t seem to be a coincidence that two of the youngest Republicans in the Senate received low scores from the Bipartisanship Index.

The average BPI score for Senate Republicans was 0.567 in 2019, considerably higher than that of Senate Democrats, which was 0.016. But when the Republican caucus is divided into two separate groups based on age (59 and below vs. 60 and above), a clear divide appears. The average BPI score for the younger group of Republican senators is 0.23, whereas the average score for the older group is 0.719.

On average, younger Republican senators are far less bipartisan than their older colleagues.  Given the pattern of older members retiring and younger members often being elected to replace them, the future of the GOP, at least in the Senate, is moving toward an increased disposition toward partisanship.

It is important to acknowledge that not all of the younger Republican senators match this trend.  Sens. Marco Rubio of Florida (#9), Dan Sullivan of Alaska (#14), and Steve Daines of Montana (#17) received scores greater than 1.0 despite being in the “59 and under” group. Likewise, Sens. Richard Shelby of Alabama (#97) and Cindy Hyde-Smith of Mississippi (#89) have scores below -0.75 despite being in the “60 and above” group. 

Another factor to consider in the continued polarization of the chamber is retirements. Already, five Republican senators up for reelection in 2022 have announced that they will not seek another term, and four of them - Sens. Pat Toomey of Pennsylvania (#44), Richard Burr of North Carolina (#26), Roy Blunt of Missouri (#21), and Rob Portman of Ohio (#4) - have positive BPI scores. Without these four senators, the average BPI score for Senate Republicans drops from 0.567 to 0.518. While a decrease of 0.049 in average BPI score may not turn heads, the loss of four, generally bipartisan senators will have a big effect on the Senate if they are replaced by highly partisan members. Portman, in particular, has been a BPI stalwart with a score of 2.254, which places him fourth among all senators. And if Sen. Chuck Grassley of Iowa (#10), decides to retire, the GOP’s BPI average would suffer a further blow.  At 87 years old, Grassley is the second-oldest person in the Senate, behind Sen. Dianne Feinstein of California.

Not only would the loss of Grassley, Portman, Blunt, Burr and Toomey in a single cycle be significant for the Republicans’ BPI score, it could also impact the Democrats. That’s because Democratic BPI scores depend on them finding bipartisan-minded partners across the aisle. If those five bipartisan GOP Senators are replaced by younger, highly partisan members, the Democrats may find they have fewer GOP bills to co-sponsor and fewer GOP co-sponsors for their own legislation.

Interestingly, the trends around age and partisanship are reversed for the Democratic Senate caucus, which recently took back control of the chamber for the first time since 2014. The average BPI score for the entire caucus is 0.016. However, when the party’s senators are divided between the two age groups (59 and under vs. 60 and above), the younger Democratic senators come out on top. The older Democratic senators average BPI score is 0.006, whereas the younger group scored 0.045. 

As with the Republicans, there are some Senate Democrats who do not fit this age pattern. Sens. Chris Murphy of Connecticut (#90) and Cory Booker of New Jersey (#92), for example, have scores below -0.75 despite being in the “59 and below” age group. Similarly, Sens. Jon Tester of Montana (#11) and Gary Peters of Michigan (#12) have scores above 1.25 despite being in the “60 and above” group. 

The cumulative effect of GOP Senate retirements, the trend toward greater partisanship among younger GOP Senators, and the potential for the Trump effect to wane now that he is out of office, raises the possibility that Senate Bipartisan Index averages might decline precipitously in the coming years, both among Republicans and the Senate as a whole. Though Senate Democrats’ scores, especially among younger members, provide some hope for the future, these general trends away from bipartisanship, driven in large part by the younger members of the Republican Senate caucus, suggest that the future of the U.S. Senate may be one where bipartisanship is an exception to the rule, rather than the rule itself.

Jerrett Alexander is an Indiana University Lugar Center Fellow.